Browse Prior Art Database

Resolving Disjoint FC-AL Networks

IP.com Disclosure Number: IPCOM000016013D
Original Publication Date: 2002-Jun-16
Included in the Prior Art Database: 2003-Jun-21
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Abstract

This disclosure relates to maintaining consistent state in storage networks composed of two or more storage adapters and one or more disks where the storage adapters are connected to the disks by more than one independent connection topology. For the purposes of this description we will consider a configuration of two FC-AL Host Bus Adapters and one dual port FC-AL disk such that the disk is connected normally to both adapters using both FC-AL disks. This is a common arrangement for FC-AL based products.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 47% of the total text.

Page 1 of 3

Resolving Disjoint FC-AL Networks

This disclosure relates to maintaining consistent state in storage networks composed of two or more storage adapters and one or more disks where the storage adapters are connected to the disks by more than one independent connection topology. For the purposes of this description we will consider a configuration of two FC-AL Host Bus Adapters and one dual port FC-AL disk such that the disk is connected normally to both adapters using both FC-AL disks. This is a common arrangement for FC-AL based products.

The network we are describing could be represented diagrammatically thus:

Loop 1

Adapter

1

Disk 1

     For simplicity of drawing each of the FC-AL loops is shows as a single line in the diagram. Using these communication paths, the adapters may be implementing some subset of the features which are associated with high function storage adapters, namely various RAID levels, caching, FlashCopy, LSA etc. It is common for these technologies to require to keep some meta-data or state information on the disks. Since both adapters have access to the disk it is important that the adapters cooperate with one another over the writing of state information onto the disk, otherwise incoherent updates to the disk's surface may result in corrupted meta-data. Typically this cooperation is achieved by sending messages between the adapters over one or both of the FC-AL loops.

     Each of the adapters has a choice of FC-AL paths through which to communicate with each other and with the disk. Two loops are used so that the network has some redundancy ; If one of the loops fails thus:

Adapter

2

Loop 2

Loop 1

Adapter

1

Disk 1

The adapters can continue to communicate over the remaining loop.

Consider however the case:

Adapter

2

Loop 2

1

[This page contains 3 pictures or other non-text objects]

Page 2 of 3

Loop 1

Adapter

1

Disk 1

     In this case, although both adapters can continue to be able to communicate with the disk, they can no longer communicate with one another. The adapters are unable to coordinate updates to the meta-data on the disk.

     One solution to this problem is the " Dual Port Resolution" or DPR algorithm. The adapters use a sector in a reserved section of the disk. This sector is known as the DPR sector. It can have 2 states:

    Not DPR assigned - Both ports of a disk are connected to an adapter. DPR Assigned - The DPR field contains the disk's port number which is allowed to access the disk. It also contains the unique identifier of the adapter that made this assignment. In the case of FC-AL this would be the adapter's World Wide Node Name (WWNN).

    When the adapter sees a disk for the first time or has to redetermine the disk metadata, it will do the following:

    Only consider ports to the disk where the maximum set of adapters can be seen. To clarify, if a disk can be seen through both ports of one adapter, but only by one port of another adapter, then only the port that both adapters can see the disk by, is to be conside...